Nose tackle Glenn Dorsey likely is out for the entirety of the 2014 season with a torn left biceps, NFL Media Insider Ian Rapoport reported Saturday, per a source informed of the injury.

Dorsey started 13 of 16 games last season, providing stout run defense on early downs.

The 49ers lack proven depth at the position, as 2013 fifth-round draft pick Quinton Dial played just 19 snaps as a rookie, per Pro Football Focus. Veteran Ian Williams started camp on the physically unable to perform list after undergoing his fourth ankle surgery since last September. He's a question mark, at best, for Week 1.

Nose tackle is a niche position in San Francisco's defense, essentially saved for obvious run situations. Coordinator Vic Fangio will have to find roughly 450 to 500 snaps at the position by turning to a Dial-Williams rotation, unearthing a street free agent, engineering a trade or slipping a couple of his defensive ends into the middle.

Whereas the 49ers offense added reinforcements this offseason, the defense has been weakened since the season-ending playoff loss at Seattle.